1. Description of the Prior Art
It is well known to make wrenches from glass fiber-reinforced plastic, in the head of which a metal insert forming a clamping jaw or gripping jaw for tightening (or loosening) is embedded. Wrenches of this type have already been described, for example, in the documents U.S. Pat. No. 2,909,954 and GB-A-1,251,419. For better absorption of the gripping stresses which are exerted at the point of contact between the insert and the nut, the use of polygonal inserts has been recommended. Although known for a very long time, these wrenches have not been developed, since, owing to the effect of the gripping forces, in time, the insert has the unfortunate tendency to turn in the head.
In the document GB-A-2,018,179, it has been suggested to provide the jaws of the opening with a thickness of metal. Unfortunately, the strength of these wrenches differs little from that of the plastic handle, which considerably limits their use.
In the document WO-84/04,721, it has therefore been suggested to cover the jaws with a U-shaped metal insert over their entire thickness. This arrangement admittedly improves the tightening torque by almost half, i.e. permits maximum tightening torques of 30 to 40 Nm to be obtained, depending on the opening of the jaws, but it does not permit the levels which are required by the industrial standards and which, for openings of 13 to 16 millimeters, are between Nm 45 to 60, to be attained.
2. Summary of the Invention
The invention eliminates these disadvantages. Its object is an improved wrench of the type in question, i.e. of plastic, which is easy to manufacture and does not have the above disadvantages. Its object is more particularly a wrench of the type in question which is light and easy to manufacture, even on a small scale, but which performs comparably with existing metal wrenches and which permits performances no worse than those of the standards currently in force to be obtained.
This plastic wrench, consisting of a handle and a gripping head which has a polygonal opening in which there is embedded an open metal insert, of hard steel and likewise polygonal, and the walls of which form jaws for tightening (or loosening), is defined in that the walls of the metal insert forming jaws are parallel to the lateral faces of the polygonal opening of each gripping head and protrude slightly from these lateral faces, and in that, in the region of the jaws, the thickness of the insert is between 0.5 and 0.7 times the thickness of the gripping head.
In other words, the invention consists in arranging the walls of the hard and resilient metal insert to be slightly protruding from the lateral faces of the opening of the head, and in having to determine an optimum ratio between the thickness of the insert and that of the actual gripping head so that, when tightening (or loosening), the stresses are essentially concentrated on the insert itself and no longer on the actual gripping head, which permits the required industrial standards of between Nm 45 to 60 to be attained.
Advantageously, in practice:
it has been determined that the larger the wrench opening, the greater the protrusion should be; thus, it has been observed that for the smallest openings, i.e. for 10 mm wrenches, this protrusion could be of the order of a few tenths of a millimeter, for example three tenths, whereas for large wrenches (32 mm wrenches), this protrusion can be of two to three millimeters; PA1 the thickness of the insert must be not less than half the thickness of the gripping head, but preferably less than 0.7 times this thickness; in fact, if this last ratio is exceeded, the residual thickness of the plastic is too small to be able to absorb properly the stresses transmitted by the insert so that, in time, cracks will appear; similarly, if this ratio is less than half, when tightening, some of the stresses are distributed in the head, which likewise becomes brittle; PA1 the material used for the insert is a metal having a coefficient of hardness of between 45 and 50 HRC, preferably in the region of 48 to 50 HRC; in fact, it has been observed that if the coefficient of hardness is less than 45, the head of the nut might, in time, cut into the walls of the insert forming the clamping jaws; on the other hand, if this hardness exceeds 55 HRC, this risk of deformation is eliminated but there is the risk that the insert might break in time; as already stated, use is preferably made of metals whose coefficient of hardness is between 48 and 50, i.e. flexible, resilient, but non-brittle metals; preferably, a steel of the type 35 CD4 or 55 S7 is used; PA1 the body (handle+gripping head) of the wrench is, as already stated, made from plastic reinforced with glass fibers or the like; plastics with a Young's modulus greater than 100 MPa are preferably used; the proportion of the glass fibers and plastic by weight increases with the opening and is at least 30% for small wrenches and may amount to 60% for large wrenches; the plastics used are preferably high-performance plastics, such as those of polyamide, preferably polyarylamide, or even of thermosetting resin, in particular vinyl ester; PA1 the faces of the insert intended to be embedded in the plastic gripping head undergo an appropriate treatment intended to improve the adhesion to plastics, such as sandblasting, serration or grooving; similarly, the adhesion between the insert and the plastic gripping head is improved by arranging in the insert through holes which, during the injection of the plastic, will fill up with the latter; PA1 the insert in plan has an irregular, generally hexagonal shape which is symmetrical relative to the median axis of the opening and in which a symmetrical opening, the walls of which form jaws, is arranged; PA1 the ratio between the large surface of the metal insert and the large surface of the plastic gripping head, excluding the gripping opening, is at least 70%, preferably of the order of 75%.
The manner in which the invention can be achieved and the advantages which ensue therefrom will become more apparent from the following exemplary embodiment, supported by the accompanying figures.